In recent years, problems due to the aging of structures such as bridges constructed during a period of high economic growth have been revealed. Since the damage would be immeasurable in the unlikely event of an accident occurring in such structures, technologies for monitoring states of these structures have been proposed up to now. For example, a technology for detecting damage in the structure in an acoustic emission (AE) method of detecting elastic waves caused by the occurrence of internal cracks or the progress of internal cracks using a high sensitivity sensor has been proposed. AE is elastic waves caused by the progress of fatigue cracks in a material. In the AE method, elastic waves are detected as an AE signal (voltage signal) by an AE sensor using a piezoelectric element. The AE signal is detected as a sign before breakage of a material occurs. Therefore, the frequency of occurrence and the signal intensity of an AE signal are used as indicators which represent the soundness of the material. For this reason, a technology for detecting an indication of the deterioration of a structure in the AE method has been studied.
As one of damage evaluation methods using an AE signal, location of the oscillation source position of elastic waves using a difference in time at which a signal arrives at a plurality of sensors is often performed. Identifying the oscillation source position of elastic waves leads to the identification of a damaged portion and is of major significance. In order to perform the location of an AE source position, it is necessary to ascertain the propagation velocity of elastic waves, but, unlike sound velocity in air, the velocity of elastic waves that propagate in solids depends on the internal structure, and there are some materials having anisotropy, and thus it is extremely difficult to ascertain an accurate velocity. A position location method based on a sound source is known as means for solving such problems. However, a detection resolution largely changes depending on an arrival direction in this method. In addition, a so-called dead band in which elastic waves cannot be completely detected depending on a direction may occur. As described above, since a range of directions in which detection is possible is limited in a conventional method, a measurement range may be limited in some cases.